From f1918806b25b04e0ede2351c8ce4cc10f59c6410 Mon Sep 17 00:00:00 2001
From: dchaofei <dchaofei@163.com>
Date: Thu, 13 Oct 2022 10:40:00 +0800
Subject: [PATCH] fix(cursor): Last needs skip empty pages

Signed-off-by: dchaofei <dchaofei@163.com>
---
 CHANGELOG.md   |  1 +
 cursor.go      | 56 ++++++++++++++++++++++++++++++++------------------
 cursor_test.go | 47 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 84 insertions(+), 20 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index cd6c0febc..61d7bc210 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@ Note that we start to track changes starting from v1.3.7.
 
 ### BoltDB
 - [Add support for loong64 arch](https://github.com/etcd-io/bbolt/pull/303).
+- Fix [the "Last" method might return no data due to not skipping the empty pages](https://github.com/etcd-io/bbolt/pull/341)
 
 ### CMD
 - [Open db file readonly in compact CLI command](https://github.com/etcd-io/bbolt/pull/292).
diff --git a/cursor.go b/cursor.go
index 98aeb449a..ba6a8345d 100644
--- a/cursor.go
+++ b/cursor.go
@@ -60,6 +60,17 @@ func (c *Cursor) Last() (key []byte, value []byte) {
 	ref.index = ref.count() - 1
 	c.stack = append(c.stack, ref)
 	c.last()
+
+	// If this is an empty page (calling Delete may result in empty pages)
+	// we call prev to find the last page that is not empty
+	for len(c.stack) > 0 && c.stack[len(c.stack)-1].count() == 0 {
+		c.prev()
+	}
+
+	if len(c.stack) == 0 {
+		return nil, nil
+	}
+
 	k, v, flags := c.keyValue()
 	if (flags & uint32(bucketLeafFlag)) != 0 {
 		return k, nil
@@ -84,26 +95,7 @@ func (c *Cursor) Next() (key []byte, value []byte) {
 // The returned key and value are only valid for the life of the transaction.
 func (c *Cursor) Prev() (key []byte, value []byte) {
 	_assert(c.bucket.tx.db != nil, "tx closed")
-
-	// Attempt to move back one element until we're successful.
-	// Move up the stack as we hit the beginning of each page in our stack.
-	for i := len(c.stack) - 1; i >= 0; i-- {
-		elem := &c.stack[i]
-		if elem.index > 0 {
-			elem.index--
-			break
-		}
-		c.stack = c.stack[:i]
-	}
-
-	// If we've hit the end then return nil.
-	if len(c.stack) == 0 {
-		return nil, nil
-	}
-
-	// Move down the stack to find the last element of the last leaf under this branch.
-	c.last()
-	k, v, flags := c.keyValue()
+	k, v, flags := c.prev()
 	if (flags & uint32(bucketLeafFlag)) != 0 {
 		return k, nil
 	}
@@ -243,6 +235,30 @@ func (c *Cursor) next() (key []byte, value []byte, flags uint32) {
 	}
 }
 
+// prev moves the cursor to the previous item in the bucket and returns its key and value.
+// If the cursor is at the beginning of the bucket then a nil key and value are returned.
+func (c *Cursor) prev() (key []byte, value []byte, flags uint32) {
+	// Attempt to move back one element until we're successful.
+	// Move up the stack as we hit the beginning of each page in our stack.
+	for i := len(c.stack) - 1; i >= 0; i-- {
+		elem := &c.stack[i]
+		if elem.index > 0 {
+			elem.index--
+			break
+		}
+		c.stack = c.stack[:i]
+	}
+
+	// If we've hit the end then return nil.
+	if len(c.stack) == 0 {
+		return nil, nil, 0
+	}
+
+	// Move down the stack to find the last element of the last leaf under this branch.
+	c.last()
+	return c.keyValue()
+}
+
 // search recursively performs a binary search against a given page/node until it finds a given key.
 func (c *Cursor) search(key []byte, pgid pgid) {
 	p, n := c.bucket.pageNode(pgid)
diff --git a/cursor_test.go b/cursor_test.go
index d2a8bc73a..1038adfbf 100644
--- a/cursor_test.go
+++ b/cursor_test.go
@@ -507,6 +507,53 @@ func TestCursor_First_EmptyPages(t *testing.T) {
 	}
 }
 
+// Ensure that a cursor can skip over empty pages that have been deleted.
+func TestCursor_Last_EmptyPages(t *testing.T) {
+	db := MustOpenDB()
+	defer db.MustClose()
+
+	// Create 1000 keys in the "widgets" bucket.
+	if err := db.Update(func(tx *bolt.Tx) error {
+		b, err := tx.CreateBucket([]byte("widgets"))
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		for i := 0; i < 1000; i++ {
+			if err := b.Put(u64tob(uint64(i)), []byte{}); err != nil {
+				t.Fatal(err)
+			}
+		}
+
+		return nil
+	}); err != nil {
+		t.Fatal(err)
+	}
+
+	// Delete last 800 elements to ensure last page is empty
+	if err := db.Update(func(tx *bolt.Tx) error {
+		b := tx.Bucket([]byte("widgets"))
+		for i := 200; i < 1000; i++ {
+			if err := b.Delete(u64tob(uint64(i))); err != nil {
+				t.Fatal(err)
+			}
+		}
+
+		c := b.Cursor()
+		var n int
+		for k, _ := c.Last(); k != nil; k, _ = c.Prev() {
+			n++
+		}
+		if n != 200 {
+			t.Fatalf("unexpected key count: %d", n)
+		}
+
+		return nil
+	}); err != nil {
+		t.Fatal(err)
+	}
+}
+
 // Ensure that a Tx can iterate over all elements in a bucket.
 func TestCursor_QuickCheck(t *testing.T) {
 	f := func(items testdata) bool {